The use of transmission lines to propagate electrical signals through multilayer ceramic packages is well known in the art. It is also well known that the electrical coupling between transmission lines may be somewhat controlled by the design of the multilayer package. Thus, design engineers take advantage of a variety of coupling techniques when they design multilayer packages, such as bandpass filters, for example.
One common coupling technique involves coupling through the proximity of the transmission lines themselves. Although coupling may be increased by moving the transmission lines closer together, practical limits may be reached due to variations in the tolerances by which transmission lines may be placed on ceramic substrates. In fact, it is not unusual to require at least a 10 mil gap (0.010 inches) due to print limitations. Other coupling techniques, such as those requiring broadside, adjacent, and alternate layer transmission lines have all been discussed in the prior art.
In a ceramic multilayer package filter design, another way to increase coupling involves incorporating a stacked coupling capacitor in the form of a printed pad between the transmission line resonators. Unfortunately, this adds both size and complexity to the filter design.
Another well known design technique involves interleaving the transmission line resonator coils so that they are overlapped on alternate layers of the multilayer ceramic package. While this technique concededly increases coupling between the resonators, it is an approach which is not very flexible and cannot be applied to non-adjacent resonators.
FIG. 1 shows a multilayer ceramic package 100 having internal transmission lines in accordance with the prior art. Referring to FIG. 1, the multilayer package is made from a series of sheets of ceramic 102 having printed transmission lines 104 printed thereon. The transmission lines 104 are connected through the package by a set of conductively coated via holes (not shown) through the sheets of ceramic 102 which interconnect the printed patterns on each layer of the package. The alignment of the vias connecting the transmission lines are shown as dashed lines 106 in FIG. 1. Significantly, the method by which the transmission lines are coupled in FIG. 1 includes a combination of broadside or proximity coupling and alternate layer coupling. It should be noted that there are no floating elements to adjust the coupling between the transmission lines in FIG. 1.
Other methods of adjusting coupling including adding external components, such as wires, chips, capacitors, or inductors to the package. However, these techniques are seldom practical alternatives for large scale, large volume manufacturing operations.
The above discussion of transmission line coupling techniques should make it clear that it is frequently difficult to get the proper coupling between transmission line resonators formed in multilayer ceramic packages. Proper coupling becomes increasingly important for many radio frequency (RF) component designs such as lowpass filters (LPF), bandpass filters (BPF), voltage controlled oscillators (VCO), directional couplers, and the like.
Thus, a floating coupling element which may be used to adjust the coupling between the transmission line structures, and which, for example, may be used to increase the capacitive coupling between transmission line resonator structures in a multilayer package, including non-adjacent resonant structures, without increasing the overall dimensions of the packages, and which increases the operable passband of multilayer bandpass filter or provides a low-side transmission zero, and which is not directly connected to any other metallized pattern including transmission lines, ground planes, or input and output ports would be considered an improvement in the art.